A 65-Myr-Long Astronomical Time Scale for the Mesozoic Deep-Sea Sequence (Inuyama, Japan): Implications for the Triassic–Jurassic Time Scale

Abstract

The astronomical time scale (ATS) for geological records is nearly complete for Cenozoic strata, and attempts have been made to extend the scale to Mesozoic and older ages. However, construction of the Mesozoic ATS is hampered by the chaotic behaviour of solar planets and the lack of a continuous pelagic sequence for the early Mesozoic due to the subduction of the ocean floor. Here, we present the ATS constructed from the early Mesozoic deep-sea sequence of the Inuyama area, central Japan. The sedimentary rhythms of bedded chert display a full range of climatic precession-related cycles: a ~20-kyr cycle as a chert–shale couplet and ~100-kyr, 405-kyr, ~2000–4000-kyr, and ~10,000-kyr (~10-Myr) cycles as chert bed thickness variations, which principally reflect changes in the burial rate of biogenic silica, most likely due to changes in silicate weathering rates. The newly established ATS tuned by the 405-kyr cycle and anchored at the end-Triassic radiolarian extinction level gives ages consistent with radiometric ages projected onto the sequence using biostratigraphy. Thus, the cyclostratigraphy of the Inuyama bedded chert provides numerical ages for the Triassic to Lower Jurassic stage boundaries, constraints on the chaotic behaviour of solar planets, and information on biogeochemical silica-cycle dynamics.